The present invention relates to an ionization deposition apparatus to be used in the surface treatment of mechanical parts or in the manufacture of semiconductors.
The ionization deposition apparatus is employed to form a highly adhesive thin film on the surface of an object. More specifically, in the ionization deposition apparatus, a solid film material is melted and evaporated to obtain a gaseous material, which is then turned to a plasma. Charged particle ions in the plasma are made to collide with high energy against an object to which a high voltage is impressed, whereby, a highly adhesive thin film is formed on the surface of the object.
Lately, a CVD method employing a gaseous film material use has been used to form high quality films in the manufacture of semiconductors.
A conventional ionization deposition apparatus will be first described with reference to FIG. 2.
As shown in FIG. 2, depicting the structure of a conventional ionization deposition apparatus, provides in a vacuum chamber 1 are an ion source A and a base material holder (holder for a to-be-deposited object) 6 placed opposed to the ion source A for holding and rotating a base material (to-be-deposited object) 7. The vacuum chamber 1 has a vacuum discharge port 2 to discharge a gas within the chamber 1 to outside the apparatus. The ion source A includes a gas introduction port 3' for introducing a gaseous film material 3, a filament 4 for generating thermoelectrons responsive to a filament current, and an anode electrode 5 for accelerating the thermoelectrons 80 as to collide against the gas molecules, thereby turning the gas molecules into a plasma. In this example, the filament 4 is connected to a filament source 8, and the anode electrode 5 is connected to an anode source 9, and the holder 6 is provided with a high voltage bias source 10.
The conventional ionization deposition apparatus of the above-described structure operates in a manner as will be discussed hereinbelow with reference to FIG. 2.
In FIG. 2, the vacuum chamber 1 is evacuated to high vacuum (approximately 10.sup.-7 Torr) by a vacuum pump (not shown) through the vacuum discharge port 2. The gaseous film material 3 of a constant flow rate is supplied from the gas introduction port 3' into the vacuum chamber 1 and, the pressure of the gaseous film material 3 in the vacuum chamber 1 is maintained to be about 10.sup.-4 -10.sup.-3 Torr. Then, a 20A-40A current is fed to the filament 4 from the filament source 8, whereby the filament 4 generates thermoelectrons. At this time, the thermoelectrons are accelerated towards the anode electrode 5 by impressing a positive voltage to the anode electrode 5 from the anode source 9. The thermoelectrons collide against the gaseous molecules of the film material gas 3 the area of the ion source A. The molecules are consequently separated into positive and negative ions, thereby generating a plasma of the gaseous film material 3 in the ion source A. In this state, when a high negative voltage is applied to the holder 6 by the high voltage bias source 10, only the positively charged ions of the gaseous film material 3 in the plasma are accelerated to collide the base material 7 with high kinetic energy. As a result, a thin film composed of the positive ions of the gaseous film material 3 is formed on the surface of the base material 7. The ions derived from the ion source A can be detected as a current (bias current) running in the bias source 10.
In the structure of the conventional apparatus as above, many ions are concentrated directly above the position where the filament 4 is located. If the base material 7 facing the filament 4 has a large area, the thickness of the film formed on the base material 7 immediately above the filament 4 becomes large, thereby causing irregularity in the distribution of the film thickness. That is, a uniform film is difficult to obtain. Moreover, since the base material 7 is heated by radiation heat from the filament 4, in the case where an amorphous film is to be formed, the adhesion between the base material 7 and the formed film is made, and the film is thus undesirably delaminated. Further, since the film gradually adheres to the surface of the filament 4 over time, the amount of generated thermoelectrons decreases even if the current flowing in the filament 4 is kept constant, and the amount of the bias current is lowered as a time elapses. A resulting drawback is that the film thickness is varied among successive lots.